Among various known processes for preparing 2,6-naphthalenedicarboxylic acid, a process comprising oxidation reaction of 2,6-dimethylnaphthalene as proposed in JP-A-49-42654 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") realizes reasonable yield but has difficulty in obtaining the starting material of high purity.
It has been proposed, on the other hand, to use 2,6-diisopropylnaphthalene which is relatively easy to synthesize and purify as a starting material of oxidation reaction for the production of 2,6-naphthalenedicarboxylic acid, and many proposals have been made to date to improve oxidation reactivity of this starting material.
For example, proposals so far made regarding oxidation reaction of 2,6-diisopropylnaphthalene with molecular oxygen in a solvent containing an aliphatic carboxylic acid in the presence of a catalyst comprising cobalt and manganese as heavy metals and a bromine compound include oxidation in the presence of an alkali metal (JP-A-61-246143, etc.), a salt of an inorganic acid, e.g., boric acid (JP-A-63-250344), potassium (JP-A-1-121240), cesium (JP-A-1-160943), or chlorine (JP-A-1-268661).
However, the process of using an alkali metal is attended by incorporation of the most part of the alkali metal into the reaction product in the form of a 2,6-naphthalenedicarboxylic acid salt and, therefore, requires removal of the alkali metal from the reaction product by, for example, neutralization with an inorganic acid. Besides, every time the catalyst is reused, an alkali metal must be added to the reaction system.
Further, the process of using an inorganic acid salt involves by-production of a large quantity of trimellitic acid.